Various species of female mosquitoes were infected with Japanese or St. Louis encephalitis Viruses either by intrathoracic inoculation or by feeding on virus-containing mixtures. The F1 progeny of these mosquitoes were tested for the presence of virus by inoculation of Toxorhynchites mosquitoes. Transovarial transmission of Japanese encephalitis virus was demonstrated in Aedes albopictus, Aedes togoi, Aedes aegypti, Armigeres subalbatus, and Culex pipiens pallens. Transovarial transmission of St. Louis encephalitis virus was not demonstrated in the experiments carried out thus far. Larvae and pupae collected in the filed in Japanese encephalitis endemic areas in Taiwan, Hong Kong, and Korea are being tested for the presence of Japanese encephalitis virus by the inoculation of Toxorhynchites mosquitoes.